# Capacitor Conceptual Question!

1. Jun 7, 2014

### victorializ

1. The problem statement, all variables and given/known data

If you increase the charge on a parallel plate capacitor from 3μC to 9μC and increase the plate separation from 1mm to 3mm, the energy stored in the capacitor changes by a factor of...

2. Relevant equations

U = 1/2QV
or 1/2CV^2

3. The attempt at a solution

I'm not sure which equation to even use, i'm not to great when it comes to differentiating between electric field and energy of the capacitor.

I think I should probably use 1/2QV but i don't have a voltage to work with so i'm confused.

Last edited: Jun 7, 2014
2. Jun 7, 2014

### dauto

Use the relation between charge Q, Voltage V, and Capacitance C to eliminate the voltage from the expression for the energy.

3. Jun 7, 2014

### victorializ

so U =1/2 Q^2/C

i'm still not sure how i would find capacitance without using the C=Q/V and not having a voltage?
also does the distance between the plates changing not matter?

4. Jun 7, 2014

### Staff: Mentor

The capacitance changes if you change the distance, and you should know a formula that connects the two.

5. Jun 7, 2014

### victorializ

oh alright i thought that the permitivity of free space had to be used when a dielectric was present only but that works thanks!